This invention relates to an inspection of a deep ultraviolet photomask (reticle) used as a light-projected master in a lithography process in the manufacture of semiconductors, and more particularly to a phase shift mask inspection apparatus for measuring the phase value of a phase shifter in a phase shift mask.
FIG. 8 shows the configuration of a phase shift mask inspection apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-151622.
The technique for finding the phase difference at the phase shift section (phase shifter) of the tested reticle has been written in Jpn. Pat. Appln. KOKAI Publication No. 4-151622.
The illumination light 2 emitted from an illumination light source 1 passes through an interference filter 3 and becomes monochromatic light having the same wavelength as that of exposure light used in the lithography process.
The light 2 passes through a polarizer 4 and becomes linearly polarized light, which is then divided by a crystal element (double refraction separation member) 5, such as a Nomarski prism or a Wollaston prism), into two luminous fluxes 2a, 2b differing in the direction of polarization of light.
These luminous fluxes 2a, 2b pass through a condenser lens 6 and illuminate a tested reticle 7 perpendicularly.
In the tested reticle 7, a transparent section 7a and a phase shift section (phase shifter) 7 are formed.
Of the two luminous fluxes 2a, 2b passed through the condenser lens 6, one luminous flux 2a passes through the transparent section 7a and the other 2b passes through the phase shifter 7b.
The two luminous fluxes 2a, 2b passed through the tested reticle 7 travel through an objective 8 and are recombined by a crystal element (double refraction separation member) 9, such as a Nomarski prism or a Wollaston prism.
The luminous flux 2 thus recombined passes through a compensator 10 and an analyzer 11 and is detected by a photoelectric sensor 12.
Interference occurs in the luminous flux 2 because, as described earlier, one luminous flux 2a has passed through the transparent section 7a while the other luminous flux 2b has passed through the phase shifter 7b.
The interference image 13 formed on an image surface varies with the phase difference between the two luminous fluxes 2a, 2b.
Therefore, a control section 14 takes in a signal corresponding to the intensity of interference outputted from the photoelectric sensor 12 and determines the phase difference at the phase shifter 7b on the basis of the signal.
The aforementioned technique, however, uses a lamp light source as the illumination light source 1, for example, and not a laser light source used to find the phase difference at the phase shifter 7b with high accuracy in the recent lithography processing.
When a laser light source is applied to the above technique, the laser light outputted from the laser light source is caused to pass through a diffusing filter so as to illuminate all of the tested reticle 7 uniformly.
Use of the diffusing filter permits speckles to take place in the two luminous fluxes 2a, 2b projected on the tested reticle 7.
This disturbs the intensity of the interference light, even when the two luminous fluxes 2a, 2b passed through the tested reticle 7 are recombined to cause interference. As a result, it is impossible to determine the phase difference at the phase shifter 7b with high accuracy.